1. Field of the Invention
This invention relates to a process for the manufacture of electronic printed circuit boards, and in particular to a continuous method for the production of such printed circuit boards.
2. The Prior Art
In the manufacture of electronic devices, such as computers, printed circuit boards are widely used to support discrete electronic components and to provide the electrical circuitry between the components. Commercial electronic computers have become more powerful since their introduction, yet they have been reduced in physical dimensions from room size to desk size. As their size has decreased and the number of interconnections due to more powerful logic has increased, the printed circuit boards used have become denser and more complex. Today's printed circuit boards can be extremely dense, with very small geometrics and with many layers.
The printed circuit boards are a composite structure having a central core typically a dielectric material such as a reinforcing cloth formed from a material such as glass fiber and a thermosetting resin such as an epoxy resin referred to in the art as a "prepreg". The term "prepreg" is derived from the fact that the reinforcing cloth, e.g. a glass fiber cloth, is preimpregnated with a volatile solution containing primarily a thermosetting resin such as an epoxy resin, catalysts and/or curing agents for the resin. The prepreg has applied on at either side thereof, metallized circuitry, usually formed of metal foil such as copper foil. The metal foil is etched or otherwise processed to provide circuits of predetermined geometrical configuration. The printed circuit board may be a two-sided composite board having circuitry on the top and bottom sides of the dielectric layer, or it may be a laminated multilayer board which also contains internal signal lines and power planes, referred to in the art as a 2s,2p structure, separated by a dielectric material. The laminated multilayer 2s,2p printed circuit boards are prepared from individual composite circuit boards which are laminated to form the multilayer structure having the etched metal circuitry sandwiched between the prepreg layers. The laminated multilayer boards are provided with through via holes and interstitial via holes to interconnect the various circuit lines and power planes.
In the manufacture of the prepreg used in the printed circuit board the reinforcing cloth suitably a glass fiber cloth is impregnated with the thermosetting resin solution, by passing the cloth through a bath of the resin solution. After removal from the impregnation bath, the prepreg is heated to remove the solvent. When the resin is an epoxy resin, the prepreg is heated at about 100.degree. to 150.degree. C. to remove the solvent and to partially cure, i.e., advance the epoxy resin to the B-stage. Thereafter, the prepreg is laminated batchwise on either side with a conductive material such as a thin, e.g. 1.0-2.0 mils thick, copper foil to form an individual circuit board, or the individual boards once the circuitry is defined, may be laminated, using the prepreg as the laminating layer to form multilayer structures.
One drawback to the prior art processes used for the manufacture of printed circuit boards is that under the conditions of manufacture, the prepreg sheet product contains a large number of voids which are formed due to air entrapment in the prepreg during the solvent removal step of the manufacturing process. The presence of voids in the printed circuit board laminate structure promotes the deterioration of laminate integrity at the interfaces between adjacent layers and leads to metal, e.g. copper, migration and the formation of short circuits between plated through holes and power planes present in the printed circuit board.
One method used by the art to eliminate voids, or entrapped air from the printed circuit board has been to subject the laminate structure immediately after formation, to heat, e.g. 125.degree. to 140.degree. C., as well as a considerable degree of compression, e.g. 200 to 700 psi for prolonged periods of up to several hours on a batchwise basis. This procedure besides being time consuming adds substantial cost to the manufacturing process.
Another method disclosed by the prior art to eliminate voids in printed circuit boards is disclosed in U.S. Pat. Nos. 4,372,800 and 4,451,317. These patents disclose a continuous method for the production of printed circuit board laminates wherein the thermosetting resin formulation used in the manufacturing process is a liquid unsaturated polyester resin which is free of volatile solvent and is capable of curing without generating liquid and gaseous byproducts.
However, a drawback to the process disclosed in U.S. Pat. Nos. 4,372,800 and 4,451,317 is that to obtain proper adhesion of the metal, e.g. copper, foil layer, to the prepreg layer of the laminate, the use of adhesives such as epoxy resins is required. This requirement has the disadvantage of another resin-resin interface, plus the disadvantage of additional coating stations during processing.
There is, therefore, a need in the art for a process to eliminate or substantially reduce voids in printed circuit boards which process is continuous and avoids the use of an adhesive layer to bond the metal foil layer to the prepreg layer.